Software Quality Insights

Mar 6 2009   6:07PM GMT

Choosing, combining agile, waterfall, other software development models

Scmconsulting Ashfaque Ahmed Profile: Scmconsulting


Having trouble deciding which software development methodology to use in your projects? Right now you have a lot of tried and tested methods for software development projects. Here are some of my rationales for choosing particular methodologies, focusly mostly on agile, waterfall, incremental iteration and continuous integration.

Compared to projects in other industries, software development projects are very different and, in most cases, more complex. For instance, in other industries, many projects are very well defined from the very beginning. All the requirements are known. How and when each task will be executed is well known in advance. The only concern on these projects is whether some purchased goods will arrive on time or not, or if the quality of some supplied part is as per specifications or not and so on.

Software development projects, on the other hand, are rarely well defined. In most cases, requirements keep changing during the entire course of software development project.

In my experience, there are some exceptions in which requirements are set and rigid. That’s when I’ve used the waterfall model. Waterfall fits when requirements are frozen, and no changes are allowed. Don’t use waterfall if the requirements in your software project are not going to be clear even after, say, 25% of the project has already been executed.

When my customers want additional requirements to be incorporated throughout the project, then I go with the agile model. Agile methods allow additional requirements and/or changes in requirements to be incorporated in any phase of the software development project, whether the project is in the design phase, building phase, testing phase or even just before the deployment phase.

Agile is great in many ways, but I’ve run into serious quality assurance (QA) issues when using agile methods. What can go wrong? Well, going back and changing design and then incorporating those changes in code — which you often do in agile development — can make the software build unstable. A quick change in design makes the design vulnerable for defects.

I’ve found that melding agile and waterfall into the incremental iteration model provides flexibility and QA in software product development. Here, requirements may come from two sources: planned release planning for the next versions of the product, and requirements coming from customers.

With the incremental interation model, we can divide all requirements into manageable groups. Then for a set of requirements, we can make a branch of the base application model and develop this branch as per requirements. We can have several branches of the base application model, and each branch will be further developed using a set of requirements. This way, each branch will have a set of requirements for which all phases of the project will be well defined. No changes in requirements are allowed in this branch until the iteration is complete. After the iteration is complete, this branch can also be merged into the main application base. If this is done, then the features which were developed in this branch become available to the main application. Otherwise, if these features are not needed in the main application then this branch will be kept separate and it will not be merged with the main application.

A slightly different model is the continuous integration model. Here instead of making branches, all the new code developed in the next release of the software goes directly to the main build.

In the continuous integration model, the software design should be based on an open standard. So whenever new requirements come, the design allows for integrating features which will fulfill these requirements. In object-oriented programming, we have parent classes based on which child classes are built. The child classes themselves will have child classes. This could go on and on, and we could end up having more than 20 layers of classes. This is fine as long as the design is open for all foreseeable requirements. The problem comes when the initial design was not kept open, and so later the design may not allow for integrating some functionality which cannot be built using the existing base design.

In most of my projects today, I use the continuous integration model. This model can be adopted for any kind of software development and is a good compromise between the rigid waterfall model and the often too-fluid concepts of agile methods. Here we are getting all the benefits of waterfall — better quality, well defined process, predictability — and the benefits of agile methods, such as incorporating new requirements quickly instead of waiting for the entire project to be completed and then adding new functionality to incorporate new requirements.

Using small iterations is good from another perspective. Testing of small iterations makes it easier to test the application. In small iterations, the testing cycle is also small. Small project sizes make all aspects of project management easier to manage.

There are many software development models from which to choose and ways to mix the best features of those models. For me, a good open design coupled with an incremental iteration model makes a good choice for projects.

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